Re: Technical question on keying of an SDR radio
Posted by
Vic Rosenthal on
Apr 09, 2019; 10:08am
URL: http://elecraft.85.s1.nabble.com/Technical-question-on-keying-of-an-SDR-radio-tp7650719p7650760.html
Oh no! NOW you tell me that all these years that I have believed that I
have been operating CW, it's really SSB!
(Disclaimer to avoid the otherwise inevitable explanations that a pure
audio tone transmitted via SSB is identical with CW: I know that).
73,
Victor, 4X6GP
Rehovot, Israel
Formerly K2VCO
CWops no. 5
http://www.qsl.net/k2vco/On 8 Apr 2019 08:24, Wayne Burdick wrote:
> Hi Phil,
>
> In the K2 the sidetone starts off as a squarewave created by the
> microcontroller. This is then shaped and injected into the AF
> amplifier. The sidetone is turned on/off at the same time as the
> carrier, which is generated by the LO (PLL synth) mixing with a gated
> TX BFO signal.
>
> In our DSP-based radios (K3/K3S and KX2/KX3), both the audio sidetone
> and the 15 kHz 2nd IF carrier are created by the DSP. Their rising
> edges are shaped mathematically using what's called a "raised cosine"
> or sigmodal waveform. We experimented to find the ideal waveform
> equation, the result being the cleaned signal possible, with
> virtually no key clicks. The DSP can of course do other things like
> apply amplitude or frequency modulation, generate voice and data mode
> signals, apply ALC and audio EQ, etc. CW is just the simplest case of
> what can be done.
>
> From the DSP, the digitized audio signal codes to a DAC (part of the
> audio CODEC IC), which then converts it to analog for injection into
> the AF amp.
>
> The 15 kHz 2nd IF carrier in the K3/K3S goes to a transmit mixer on
> the KREF4 module where it's up-converted to about 8215 kHz. It is
> then routed to the first IF mixer. The 8215 kHz signal passes through
> two crystal filters enroute mixer. In the KX2/KX3, the 15 kHz IF
> signal is converted to a pair of IQ signals (90 degrees out of phase
> with each other) to directly modulate a quadrature up-converter. The
> other input to the up-converter is a pair of balanced LO signals,
> again separated by 90 degrees. A result of IQ modulation is that one
> sideband is cancelled out, resulting in a single RF carrier at the
> desired frequency.
>
> In all cases, the rest of the T/R sequencing involves the usual
> amplifiers, PIN diodes, filter switching, etc.
>
> 73, Wayne N6KR
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